The philosophical arguments in the abortion debate are deontological or rights-based. The view that all or almost all abortion should be illegal generally rests on the claims: (1) that the existence and moral right to life of human beings (human organisms) begins at or near conception-fertilization; (2) that induced abortion is the deliberate and unjust killing of the embryo in violation of its right to life; and (3) that the law should prohibit unjust violations of the right to life. The view that abortion should in most or all circumstances be legal generally rests on the claims: (1) that women have a right to control what happens in and to their own bodies; (2) that abortion is a just exercise of this right; and (3) that the law should not criminalize just exercises of the right to control one's own body and its life-support functions.

Although both sides are likely to see the rights-based considerations as paramount, some popular arguments appeal to consequentialist or utilitarian considerations. For example, pro-life advocacy groups (see the list below) sometimes claim the existence of post-abortion syndrome or a link between abortion and breast cancer, alleged medical and psychological risks of abortion. On the other side, pro-choice groups (see the list below) say that criminalizing abortion will lead to the deaths of many women through "back-alley abortions"; that unwanted children have a negative social impact (or conversely that abortion lowers the crime rate); and that reproductive rights are necessary to achieve the full and equal participation of women in society and the workforce. Consequentialist arguments on both sides tend to be vigorously disputed, though are not widely discussed in the philosophical literature.

Contemporary philosophical literature contains two kinds of arguments concerning the morality of abortion. One family of arguments (see the following three sections) relates to the moral status of the embryo—the question of whether the embryo has a right to life, is the sort of being it would be seriously wrong to kill, or in other words is a "person" in the moral sense. An affirmative answer would support claim (1) in the central pro-life argument, while a negative answer would support claim (2) in the central pro-choice argument.

Another family of arguments (see the section on Thomson, below) relates to bodily rights—the question of whether the woman's bodily rights justify abortion even if the embryo has a right to life. A negative answer would support claim (2) in the central pro-life argument, while an affirmative answer would support claim (2) in the central pro-choice argument.

Mary Anne Warren, in her article arguing for the permissibility of abortion,[1] holds that moral opposition to abortion is based on the following argument:

It is wrong to kill innocent human beings.

The embryo is an innocent human being.

Hence it is wrong to kill the embryo.

Warren, however, thinks that "human being" is used in different senses in (1) and (2). In (1), "human being" is used in a moral sense to mean a "person", a "full-fledged member of the moral community". In (2), "human being" means "biological human". That the embryo is a biologically human organism or animal is uncontroversial, Warren holds. But it does not follow that the embryo is a person, and it is persons that have rights, such as the right to life.[2]

To help make a distinction between "person" and "biological human", Warren notes that we should respect the lives of highly intelligent aliens, even if they are not biological humans. She thinks there is a cluster of properties that characterize persons:[3]

consciousness (of objects and events external and/or internal to the being), and in particular the capacity to feel pain

reasoning (the developed capacity to solve new and relatively complex problems)

self-motivated activity (activity which is relatively independent of either genetic or direct external control)

the capacity to communicate, by whatever means, messages of an indefinite variety of types, that is, not just with an indefinite number of possible contents, but on indefinitely many possible topics

the presence of self-concepts, and self-awareness, either individual or racial, or both

A person does not have to have each of these, but if something has all five then it definitely is a person whether it is biologically human or not, while if it has none or perhaps only one then it is not a person, again whether it is biologically human or not. The fetus has at most one, consciousness (and this only after it becomes susceptible to pain—the timing of which is disputed), and hence is not a person.[4]

Other writers apply similar criteria, concluding that the embryo lacks a right to life because it lacks self-consciousness,[5] or rationality and self-consciousness,[6] or "certain higher psychological capacities" including "autonomy".[7]

Others conclude that personhood should be based on "brain birth" concept, which is in essence the reversal of the brain death used as a modern definition of medical death. Under this proposal, presence of brain waves would be enough to grant personhood, even with other features lacking. Based on whether brain activity in the brain stem, or just in the cerebral cortex, is relevant for personhood, two concepts of "brain birth" emerge:[8]

These writers disagree on precisely which features confer a right to life,[10] but agree those features must be certain developed psychological or physiological features which the embryo lacks.

Warren's arguments face two main objections. The comatose patient objection claims that as patients in a reversible coma do not satisfy Warren's (or some other) criteria—they are not conscious, do not communicate, and so on—therefore they would lack a right to life on her view.[11] One response is that "although the reversibly comatose lack any conscious mental states, they do retain all their unconscious [or dispositional] mental states, since the appropriate neurological configurations are preserved in the brain."[12] This may allow them to satisfy some of Warren's criteria. The comatose also still possess brain activity (brain waves), so this objection does not apply to "brain birth" theories. Finally, there are some post-natal humans who are unable to feel pain due to genetic disorders and thus do not satisfy all of Warren's criteria.[13]

The infanticide objection points out that infants (indeed up to about one year of age, since it is only around then that they begin to outstrip the abilities of non-human animals) have only one of Warren's characteristics—consciousness—and hence would have to be accounted non-persons on her view; thus her view would permit not only abortion but infanticide. Warren agrees that infants are non-persons (and so killing them is not strictly murder), but denies that infanticide is generally permissible.[14] For, Warren claims, once a human being is born, there is no longer a conflict between it and the woman's rights, since the human being can be given up for adoption. Killing such a human being would be wrong, not because it is a person, but because it would go against the desires of people willing to adopt the infant and to pay to keep the infant alive.

Nonetheless, Warren grants that her argument entails that infanticide would be morally acceptable under some circumstances, such as those of a desert island. Philosopher Peter Singer similarly concludes that infanticide, particularly of severely disabled infants, is justifiable under certain conditions.[15] And Jeff McMahan grants that under very limited circumstances it may be permissible to kill one infant to save the lives of several others.[16] Opponents may see these concessions as a reductio ad absurdum of these writers' views; while supporters may see them merely as examples of unpleasant acts being justified in unusual cases.

Since brain waves appear in the lower brain (brain stem) in 6–8 weeks of gestation, and in the higher brain (cerebral cortex) in 22–24 weeks of gestation, both "whole brain" and "higher brain" brain birth personhood concepts based on the presence of brain waves do not permit infanticide.[8]

Some opponents of Warren's view believe that what matters morally is not that one be actually exhibiting complex mental qualities of the sort she identifies, but rather that one have in oneself a self-directed genetic propensity or natural capacity to develop such qualities. In other words, what is crucial is that one be the kind of entity or substance that, under the right conditions, actively develops itself to the point of exhibiting Warren's qualities at some point in its life, even if it does not actually exhibit them because of not having developed them yet (embryo, infant) or having lost them (severe Alzheimer's). Because human beings do have this natural capacity—and indeed have it essentially—therefore (on this view) they essentially have a right to life: they could not possibly fail to have a right to life.[17] Further, since modern embryology shows that the embryo begins to exist at conception and has a natural capacity for complex mental qualities, therefore the right to life begins at conception.

Grounding the right to life in essential natural capacities rather than accidental developed capacities is said to have several advantages.[18] As developed capacities are on a continuum, admitting of greater and lesser degrees—some, for example, are more rational and self-conscious than others—therefore: (1) the "developed capacities" view must arbitrarily select some particular degree of development as the cut-off point for the right to life—whereas the "natural capacities" view is non-arbitrary; (2) those whose capacities are more developed would have more of a right to life on the 'developed capacities' view—whereas the "natural capacities" view entails we all have an equal right to life; and (3) the continuum of developed capacities makes the exact point at which personhood ensues vague, and human beings around that point, say between one and two years of age, will have a shadowy or indeterminate moral status—whereas there is no such indeterminacy on the "natural capacities" view.[19]

Some defenders of Warren-style arguments grant that these problems have not yet been fully solved,[20] but reply that the "natural capacities" view fares no better. It is argued, for example, that as human beings vary significantly in their natural cognitive capacities (some are naturally more intelligent than others), and as one can imagine a series or spectrum of species with gradually diminishing natural capacities (for example, a series from humans down to amoebae with only the slightest differences in natural capacities between each successive species), therefore the problems of arbitrariness and inequality will apply equally to the "natural capacities" view.[21] In other words, there is a continuum not only of developed but of natural capacities, and so the "natural capacities" view will inevitably face these problems as well.

Some critics reject the "natural capacities" view on the basis that it takes mere species membership or genetic potential as a basis for respect (in essence a charge of speciesism),[22] or because it entails that anencephalic infants and the irreversibly comatose have a full right to life.[23] Moreover, as with Marquis's argument (see below), some theories of personal identity would support the view that the embryo will never itself develop complex mental qualities (rather, it will simply give rise to a distinct substance or entity that will have these qualities), in which case the "natural capacities" argument would fail. Respondents to this criticism argue that the noted human cases in fact would not be classified as persons as they do not have a natural capacity to develop any psychological features.[24][25][26]

A seminal essay by Don Marquis argues that abortion is wrong because it deprives the embryo of a valuable future.[27] Marquis begins by arguing that what makes it wrong to kill a normal adult human being is the fact that the killing inflicts a terrible harm on the victim. The harm consists in the fact that "when I die, I am deprived of all of the value of my future":[28] I am deprived of all the valuable "experiences, activities, projects, and enjoyments" that I would otherwise have had.[29] Thus, if a being has a highly valuable future ahead of it—a "future like ours"—then killing that being would be seriously harmful and hence seriously wrong.[30] But then, as a standard embryo does have a highly valuable future, killing it is seriously wrong.[31] And so "the overwhelming majority of deliberate abortions are seriously immoral", "in the same moral category as killing an innocent adult human being".[32]

A consequence of this argument is that abortion is wrong in all the cases where killing a child or adult with the same sort of future as the embryo would be wrong. So for example, if involuntary euthanasia of patients with a future filled with intense physical pain is morally acceptable, aborting embryos whose future is filled with intense physical pain will also be morally acceptable. But it would not do, for example, to invoke the fact that some embryo's future would involve such things as being raised by an unloving family, since we do not take it to be acceptable to kill a five-year-old just because her future involves being raised by an unloving family. Similarly, killing a child or adult may be permissible in exceptional circumstances such as self-defense or (perhaps) capital punishment; but these are irrelevant to standard abortions.

Marquis's argument has prompted several objections. The contraception objection claims that if Marquis's argument is correct, then, since sperm and ova (or perhaps a sperm and ovum jointly) have a future like ours, contraception would be as wrong as murder; but as this conclusion is (it is said) absurd—even those who believe contraception is wrong do not believe it is as wrong as murder—the argument must be unsound. One response[33] is that neither the sperm, nor the egg, nor any particular sperm-egg combination, will ever itself live out a valuable future: what will later have valuable experiences, activities, projects, and enjoyments is a new entity, a new organism, that will come into existence at or near conception; and it is this entity, not the sperm or egg or any sperm-egg combination, that has a future like ours.

As this response makes clear, Marquis's argument requires that what will later have valuable experiences and activities is the same entity, the same biological organism, as the embryo.[34] The identity objection rejects this assumption. On certain theories of personal identity (generally motivated by thought experiments involving brain or cerebrum transplants), each of us is not a biological organism but rather an embodied mind or a person (in John Locke's sense) that comes into existence when the brain gives rise to certain developed psychological capacities.[35] If either of these views is correct, Marquis's argument will fail; for the embryo (even the early fetus, lacking the relevant psychological capacities) would not itself have a future of value, but would merely have the potential to give rise to a different entity, an embodied mind or a person, that would have a future of value. The success of Marquis's argument thus depends on one's favored account of personal identity.

The interests objection claims that what makes murder wrong is not just the deprivation of a valuable future, but the deprivation of a future that one has an interest in. The embryo has no conscious interest in its future, and so (the objection concludes) to kill it is not wrong. The defender of Marquis-style arguments may, however, give the counterexample of the suicidal teenager who takes no interest in his or her future, but killing whom is nonetheless wrong and murder.[36] If the opponent responds that one can have an interest in one's future without taking an interest in it, then the defender of the Marquis-style argument can claim that this applies to the embryo.[37] Similarly, if an opponent claims that what is crucial is having a valuable future which one would, under ideal conditions, desire to preserve (whether or not one does in fact desire to preserve it),[38] then the defender may ask why the embryo would not, under ideal conditions, desire to preserve its future.

The equality objection claims that Marquis's argument leads to unacceptable inequalities.[39] If, as Marquis claims, killing is wrong because it deprives the victim of a valuable future, then, since some futures appear to contain much more value than others—a 9 year old has a much longer future than a 90 year old, a middle class person's future has much less gratuitous pain and suffering than someone in extreme poverty—some killings would turn out to be much more wrong than others. But as this is strongly counterintuitive (most people believe all killings are equally wrong, other things being equal), Marquis's argument must be mistaken. Some writers have concluded that the wrongness of killing arises not from the harm it causes the victim (since this varies greatly among killings), but from the killing's violation of the intrinsic worth or personhood of the victim.[40] However, such accounts may themselves face problems of equality,[41] and so the equality objection may not be decisive against Marquis's argument.

The psychological connectedness objection claims that a being can be seriously harmed by being deprived of a valuable future only if there are sufficient psychological connections—sufficient correlations or continuations of memory, belief, desire and the like—between the being as it is now and the being as it will be when it lives out the valuable future.[42] As there are few psychological connections between the embryo and its later self, it is concluded that depriving it of its future does not seriously harm it (and hence is not seriously wrong). A defence of this objection is likely to rest, as with certain views of personal identity, on thought experiments involving brain or cerebrum swaps; and this may render it implausible to some readers.

In her well-known article "A Defense of Abortion", Judith Jarvis Thomson argues that abortion is in some circumstances permissible even if the embryo is a person and has a right to life, because the embryo's right to life is overtrumped by the woman's right to control her body and its life-support functions. Her central argument involves a thought experiment. Imagine, Thomson says, that you wake up in bed next to a famous violinist. He is unconscious with a fatal kidney ailment; and because only you happen to have the right blood type to help, the Society of Music Lovers has kidnapped you and plugged your circulatory system into his so that your kidneys can filter poisons from his blood as well as your own. If he is disconnected from you now, he will die; but in nine months he will recover and can be safely disconnected. Thomson takes it that you may permissibly unplug yourself from the violinist even though this will kill him. The right to life, Thomson says, does not entail the right to use another person's body, and so in disconnecting the violinist you do not violate his right to life but merely deprive him of something—the use of your body—to which he has no right. Similarly, even if the fetus has a right to life, it does not have a right to use the pregnant woman's body and life-support functions against her will; and so aborting the pregnancy is permissible in at least some circumstances. However, Thomson notes that the woman's right to abortion does not include the right to directly insist upon the death of the child, should the fetus happen to be viable, that is, capable of surviving outside the womb.[43]

Critics of this argument generally agree that unplugging the violinist is permissible, but claim there are morally relevant disanalogies between the violinist scenario and typical cases of abortion. The most common objection is that the violinist scenario, involving a kidnapping, is analogous only to abortion after rape. In most cases of abortion, it is said, the pregnant woman was not raped but had intercourse voluntarily, and thus has either tacitly consented to allowing the embryo to use her body (the tacit consent objection[44]), or else has a duty to sustain the embryo because the woman herself caused it to stand in need of her body (the responsibility objection[45]). Other common objections turn on the claim that the embryo is the pregnant woman's child whereas the violinist is a stranger (the stranger versus offspring objection[46]); that abortion kills the embryo whereas unplugging the violinist merely lets him die (the killing versus letting die objection[46]); or, similarly, that abortion intentionally causes the embryo's death whereas unplugging the violinist merely causes death as a foreseen but unintended side-effect (the intending versus foreseeing objection;[47] cf the doctrine of double effect).

Defenders of Thomson's argument—most notably David Boonin[48]—reply that the alleged disanalogies between the violinist scenario and typical cases of abortion do not hold, either because the factors that critics appeal to are not genuinely morally relevant, or because those factors are morally relevant but do not apply to abortion in the way that critics have claimed. Critics have in turn responded to Boonin's arguments.[49]

Alternative scenarios have been put forth as more accurate and realistic representations of the moral issues present in abortion. John Noonan proposes the scenario of a family who was found to be liable for frostbite finger loss suffered by a dinner guest whom they refused to allow to stay overnight, although it was very cold outside and the guest showed signs of being sick. It is argued that just as it would not be permissible to refuse temporary accommodation for the guest to protect them from physical harm, it would not be permissible to refuse temporary accommodation of a fetus.[50]

Other critics claim that there is a difference between artificial and extraordinary means of preservation, such as medical treatment, kidney dialysis, and blood transfusions, and normal and natural means of preservation, such as gestation, childbirth, and breastfeeding. They argue that if a baby was born into an environment in which there was no replacement available for her mother's breast milk, and the baby would either breastfeed or starve, the mother would have to allow the baby to breastfeed. But the mother would never have to give the baby a blood transfusion, no matter what the circumstances were. The difference between breastfeeding in that scenario and blood transfusions is the difference between gestation and childbirth on the one hand, and using your body as a kidney dialysis machine on the other.[51][52][53][54][55][56]

^Warren 1973: 457. The same point is made in Tooley 1972: 40-43; Singer 2000: 126-28 and 155-156; Pojman 1994: 280; and elsewhere. Note that "person" can also be used in two senses. In John Locke's sense (often employed in discussions of personal identity), "person" is a descriptive term that tells us about a being's psychological properties. In Warren's sense, "person" is a moral or evaluative term that tells us about a being's moral properties. Warren and others hold, however, that being a person in the moral sense actually requires being a person in the psychological sense.

^Warren 1973: 458. Glover 1977:127 and English 1975: 316-317 also refer to a 'cluster' of properties as constituting personhood.

^Michael Tooley argues that the bearer of a right to life must conceive of itself "as a continuing subject of experiences and other mental states" (Tooley 1972: 44), or must at some time possess "the concept of a continuing self or mental substance" (Tooley 1984: 218)

^On the pro-life side (see below), it is similarly unclear which features one must have a natural capacity for, in order to have a right to life (cf Schwarz 1990: 105-109), or which features constitute a "future like ours".

^Grisez 1970: 277-287; Lee 1996 and 2004; Lee and George 2005: 16-20; Schwarz 1990: 91-93; Beckwith 1993: 108-10; Reichlin 1997: 22-23; and many others. Note that on Marquis's view (see below), by contrast, one could fail to have a right to life—for example by becoming irreversibly comatose, since one's future would then lack valuable experiences and activities.

^Marquis 1989: 183. Although Marquis views the killing of an embryo or normal human adult as seriously wrong, he avoids any reference to "rights" or the "right to life", and so is apparently not committed to deontological ethics.